Pharmacologic coronary vasodilation in conjunction with myocardial perfusion scintigraphy has become an alternative to dynamic exercise test for the diagnosis and risk stratification of coronary artery disease, especially in patients who are unable to perform adequate exercise. Dipyridamole and adenosine have been used for pharmacologic stress testing with myocardial perfusion imaging. Adenosine is a potent coronary vasodilator with rapid onset of action, short half-life, near maximal coronary vasodilation and less serious side effects. ST segment depression has been reported in about 7-15% of patients with coronary artery disease receiving dipyridamole in conjunction with myocardial perfusion imaging. The exact cause and clinical significance are not known. In order to evaluate the relationship between adenosine-induced ST segment depression during $^{99m}Tc$-MIBI myocardial perfusion scintigraphy and the severity of coronary artery disease, we performed $^{99m}Tc$-MIBI imaging after intravenous Infusion of adenosine In 120 patients with suspected coronary artery disease. Of the 120 patients, 28 also performed coronary angiography. There were 24 patients with ST segment depression during $^{99m}Tc$-MIBI scintigraphy and 96 patients without ST segment depression. Adenosine was infused Intravenously at a dose of 0.14mg/kg per minute lot 6minutes and $^{99m}Tc$-MIBI was injected at 3 minute. We then com-pared the hemodynamic changes, side effects, scintigraphic and angiographic findings. Heart rate increased $90{pm}19$ beats/minute in the group with ST depression compared with $80{pm}16$ beats/minute in the group without ST depression(p<0.05). Baseline systolic blood pressure was significantly higher in the group with ST depression($152{pm}27$ mmHg) than in the group without 57 depression($140{pm}21$mmHg, p<0.05). Double product at baseline($10.90{pm}2.77$ versus $9.55{pm}2.34;beats/minute{ imes}mmHg$) and during adenosine infusion($12.72{pm}3.89$ versus $10.83{pm}2.98;beats/minute{ imes}mmHg$) were significantly higher in the group with ST depression(p<0.05). The incidence of anginal chest pain was also significantly higher in the group with ST depression(ST versus 29%, p<0.0001). The $^{99m}Tc$-MIBI images were abnormal in 23(96%) patients with ST segment depression and 66(69%) patients without ST segment depression(p<0.05). In patients with ST segment depression, there were more reversible perfusion defects than in patients without ST segment depression(83 versus 55%, p<0.05). The number of abnormal segments were significantly higher in the group with ST depression($3.05{pm}2.01$ versus $1.51{pm}1.45$, p<0.005). In patients with ST segment depression, there were more segments of reversible perfusion defects than in patients without segment depression($2.15{pm}2.11$ versus $0.89{pm}1.24$, p<0.05). There were no differences in the angiographic severity by vessel(p ; NS). We concluded that ST segment depression during $^{99m}Tc$-MIBI myocardial perfusion scintigraphy with Intravenous adenosine is related to the severity of coronary artery disease.